The present invention relates to a method and apparatus for providing a web of thermoplastic filaments. In particular, the present invention relates to a method and apparatus whereby a slot member is combined with an accelerator gun type tube system.
Through a typical spunbonding process, nonwoven fabrics are made by depositing spun filaments on a moving support to form a web followed by bonding the web. Generally, the thermoplastic filaments are first continuously extruded, quenched, drawn, and attenuated by exposure to a high velocity fluid, and then deposited on the moving support. The quality of the final web depends to a great degree on the relative arrangement and uniformity of the filaments in the deposited web. The method and apparatus used for drawing and attenuating the filaments will determine their uniformity and arrangement in the web.
Manufacturers of non-wovens have made many innovations regarding such drawing and attenuating methods and apparatuses. Generally, good separation between filaments in combination with a high velocity for high rates of production is desired. When separation between filaments is poor, strands undesirably become wrapped about one another; a condition referred to as xe2x80x9cropinessxe2x80x9d. Ropiness in turn leads to poor uniformity in the resultant web.
Early spunbonding processes typically utilized round tubes for drawing and attenuating the filaments. High velocities of air were sent through the tubes to carry the filaments through and direct them for depositing on a moving support below to form a web. These tubes may be referred to as xe2x80x9cVenturi tubesxe2x80x9d or xe2x80x9cattenuator gunsxe2x80x9d, with the xe2x80x9cLurgixe2x80x9d process one system of this type (use herein of any of these terms is generally intended to be interchangeable). Systems such as these have several problems associated with them. xe2x80x9cGun outsxe2x80x9d can occur when one or more filaments break at the entry point of a tube due to the aggressive handling created by the air acceleration in the tube. When a gun out occurs, unattenuated fibers and polymer drips can fall directly onto the support below. This can result in an expensive total line stoppage for cleaning of the support.
Additional problems are also associated with Lurgi tube systems. Inside the tubes the filaments make contact with other filaments, which may result in them becoming stuck together or entangled, causing a defect referred to as xe2x80x9cropinessxe2x80x9d. Additionally, the interior of the tubes can become partially or completely blocked due to polymer deposits resulting from drips or filaments contacting the tube inner surface. Finally, the high velocity of the air in the tubes is associated with a general non-uniform or splotchy appearance to the finished web, as the filaments are directed to the moving support with considerable force, providing little opportunity for spreading or randomization. These various problems tend to result in undesirable webs with low tensile strength.
Many attempts have been made to improve the performance of Lurgi tubes. For example, U.S. Pat. Nos. 3,163,753; 3,341,394; and 4,009,508 describe the use of corona electrodes for charging the filaments electrostaticly in combination with use of a Lurgi tube. Electrostatic charging tends to separate the filaments and thus to decrease ropiness and to otherwise result in a more uniform web. Although this method provides some improvement, it does not achieve fully satisfactory performance.
Alternate drawing and attenuation methods and apparatuses have also been developed. In particular, U.S. Pat. Nos. 4,340,563 and 4,405,297 describe slot draw processes which replace Lurgi tubes with an elongated slot extending across substantially the whole web cross section to draw and attenuate filaments. Further advancements to the slot draw system are described in U.S. Pat. Nos. 4,578,134; 5,397,413; 5,545,371. These patents describe modifications to the slot draw process that include, respectively, a complex directed air stream, a corona treatment, and an air stream with water dispersed in it. The slot draw system in its various forms has generally resulted in improved filament drawing and attenuating for more uniform and efficient non-woven web production
Lurgi tubes, however, remain in use for economic and performance related reasons. Because of the high velocity of air that is developed through the tube, tube attenuation offers superior filament crystallinity and orientation over slot draw systems, which have a lower air velocity and resultant lower crystallinity. Low crystallinity can result in low filament tenacity, and in filament shrinkage during subsequent thermal bonding. Also, because of the large capital investment required to install a slot draw system, it is not practical for many manufacturers having existing Lurgi tube systems to take advantage of the advances offered by a slot draw system.
There is therefore a need for improved Lurgi tube systems.
Prior efforts at improving such systems have included deflecting the filaments off of a deflector plate mounted below the tube outlet. While this method broadened the laydown pattern of the filaments in the web, good uniformity was not realized. Another effort included mechanically oscillating one or more of the tubes, deflector plates, and/or the web. These methods, however, were found to depend on the operating frequency of the oscillating member(s), and also entailed numerous related maintenance problems.
Another proposed improvement to the Lurgi tube systems is described in U.S. Pat. No. 5,225,018 to Zeldin et al. Zeldin attaches a pair of tapered guide plates to a row of Lurgi tubes to form a slot at the outlet of tubes prior to depositing filament on the web. The plates are slanted at a precise angle so that turbulent air flow is developed in the tapered slot formed between the plates. As the filaments exit the tubes and enter the tapered slot, they are randomly spread by the turbulent air flow that exists in the slot. Zeldin further includes corona means below the outlet of the tapered slot for additional filament separation. Zeldin""s device results in relatively randomly deposited filaments across the width of the web, with a high ratio of cross direction filament depositing to machine direction filament depositing.
The apparatus of Zeldin, however, leaves many problems with using Lurgi tubes unresolved. Due to the high cross direction machine direction ratio that webs have when produced using the apparatus of Zeldin, for instance, Lurgi tubes are still unable to be used for production of a relatively uniform web with a high machine direction orientation and low machine direction elongation.
There is therefore an unresolved need in industry for an improved apparatus and method for producing a uniform web of continuous filaments with high machine direction strength, and high crystallinity.
It is an object of the invention to provide an apparatus and method for the production of a uniform web of filaments having high crystallinity, and high machine direction orientation.
The present invention generally comprises an apparatus and a method for providing a uniform web of non-woven filaments having high crystallinity, high machine direction orientation, and low machine direction elongation.
The apparatus of the present invention generally comprises filament transport means having an exit, with a confined zone of reduced turbulence adjacent to the transport means exit. As the filaments exit the transport means conveyed through the confined zone where the conveying fluid velocity slows and turbulence is reduced, thus spreading the filaments in a cross direction.
The transport means preferably comprise a plurality of Lurgi tubes, while the preferred confined zone comprises a delivery slot defined by a slot member. The tube exits are located adjacent to the elongated delivery slot. The preferred delivery slot is defined by the substantially parallel first and second sideplates and substantially parallel endplates of the slot member. The sideplates and endplates are also parallel to the transport means. Preferably, a tapered transition member is between the tube exit and delivery slot entrance to aid in introduction of the filaments to the slot. The high velocity fluid flow coming out of the tubes slows in the larger slot, allowing for spreading of the filaments and discouraging ropiness. As the filaments travel through the tapered transition member, they are spread in a cross direction along the elongated delivery slot. Because the sidewalls are substantially parallel to one another and to the Lurgi tubes, the filaments travel through the delivery slot without significant machine direction deflection, thereby minimizing randomization (herein defined as machine direction deflection) and providing high machine direction orientation.
The preferred apparatus of the invention further comprises electrostatic charging means in one of the first or second side plates for electrostatically charging the filaments with like charges as they travel through the delivery slot. The effect of having like surface charges on the filaments is to have the filaments physically repel one another. The charged filaments are thus further separated, thereby discouraging and even correcting filament ropiness. Preferred electrostatic charging means comprise corona discharge pins in one of the first or second sideplates extending along the length of the plate.
The preferred discharge pins are positioned inside the throat of the slot, so that the filaments are spread while still in the slot. In this manner, the majority machine direction orientation of the filament curtain is maintained, rather than encouraging a cross direction randomization that would result should the filaments be charged after exiting the slot.
The apparatus of the invention may further preferably comprise means for pneumatically carrying filament through the transport means and delivery slot, a moving support member traveling below the delivery slot to receive the deposited web, and bonding means for bonding the deposited web.
The method of the present invention comprises the sequential steps of transporting filaments through transport means, which preferably comprise Lurgi tubes, and transporting them through a confined zone of reduced turbulence. The filaments are spread in a cross direction as they travel through the confined zone. The preferred confined zone comprises a delivery slot defined by substantially parallel first and second sidewalls, which are also substantially parallel to the Lurgi tubes. The filaments thus travel through the preferred delivery slot with minimal randomization, while spreading uniformly into a filament curtain in a cross direction. Preferably, the filaments are transported through a tapered transition member between the transport means and the delivery slot for further cross direction spreading of the filament curtain. Also, the filaments are preferably electrostatically charged with like charges while traveling through the delivery slot so that the individual filaments will repel one another and be further separated both within the slot and while traveling between the slot and the moving support below.
The filaments are preferably transported pneumatically by air through the Lurgi tube and delivery slot. Also, the method of the invention further comprises preferred steps of depositing the web on a moving support below the slot, and subsequently bonding the deposited web.
Using the method and apparatus of the invention, a significantly improved cross direction spreading of filaments is achieved as compared to use of Lurgi tubes alone. Because of the substantially parallel apparatus side and end plates, minimal filament deflection in the machine direction occurs through the delivery slot. The method and apparatus of the present invention thus result in a deposited web with high machine direction orientation and low machine direction elongation, while having greatly improved cross direction uniformity over use of Lurgi tubes alone. This is a result not possible using a tapered slot. Further, the high fluid velocity possible in the tube transport of the present invention results in improved filament crystallinity over conventional slot draw attenuators. The method and apparatus of the present invention therefore resolves a heretofor unresolved need in industry in a facile and efficient manner.
The above brief description sets forth rather broadly the more important features of the present disclosure so that the detailed description that follows may be better understood, and so that the present contributions to the art may be better appreciated. There are, of course, additional features of the disclosure that will be described hereinafter which will form the subject matter of the claims appended hereto. In this respect, before explaining the embodiments of the disclosure in detail, it is to be understood that the disclosure is not limited in its application to the details of the construction and the arrangements set forth in the following description or illustrated in the drawings. The present invention is capable of other embodiments and of being practiced and carried out in various ways, as will be appreciated by those skilled in the art. Also, it is to be understood that the phraseology and terminology employed herein are for description and not limitation.